Magnetic equalization of sensitivity in ribbon microphone assemblies



July 19, 1949.

L. J. ANDERSON 2,476,396

MAGNETIC EQUALIZATION OF SENSITIVITY IN RIBBON MICROPHONE ASSEMBLIES Original Filed Dec. 28, 1959 iyz J Zmnentor w/ LL,

Gttorneg Patented July 19;1949

MAGNETIC EQUALIZATION OF SENSITIVITY IN RIBBON MICROPHONE ASSEMBLIES Leslie J. Anderson, Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Original application December 28, 1939, Serial No. 311,371. Divided and this application April 29, 1942, Serial No. 440,909

4 Claims.

This invention relates to electroacoustical apparatus and more particularly to a uni-directional microphone, the present application being a division of my copending application Serial No. 311,371, filed December 28, 939, now Patent No. 2,295,376, granted September 8, 1942.

In the case of uni-directional microphones employing a ribbon or other conductor mounted for movement in a magnetic field and having a section responsive to the pressure component of a sound wave and another section responsive to the pressure gradient component of the sound wave, as shown, for example, in the Olson and Weinberger Reissue Patent 19,115, the reqirements of fundamental resonant frequency dictate a certain minimum length of ribbon consistent with proper mechanical design. This minimum usually lies in the order of one inch. When the acoustic line or pipe is coupled to the pressure responsive section of the ribbon, a considerable mismatch in sensitivity between the pressure responsive section and the pressure gradient, or velocity, responsive section may result. This is especially likely to be so if the acoustic line or pipe is of small physical size; that is to say, of small diameter.

The conventional method of dealing with this situation is to retain the mechanical length of the ribbon in the velocity section, but make a portion of the velocity section inactive by extending it beyond the magnetic field. This, of course, assumes that the pressure responsive section of the ribbon has the lower sensitivity, which is usually the case. Extending a portion of the ribbon beyond the air gap or magnetic field is inefiicient since it is equivalent to inserting a resistor in series with the active portion of the velocity section.

The primary object of my present invention is to provide an improved uni-directional microphone wherein the aforementioned difliculty is obviated.

More particularly, it is an object of my present invention to provide an improved uni-directional microphone of the type set forth which will have improved sensitivity.

Another object of my present invention is to provide an improved uni-directional microphone asaforementioned which can be adjusted easily and quickly to provide maximum sensitivity consistent with a desired directional characteristic.

It is also an object of my present invention to provide an improved uni-directional microphone as aforementioned which is simple in construction, inexpensive in cost, and highly eificient in use.

In accordance with the present invention, I provide for the conductive elements or ribbon sections of the microphone a field structure which supplies to the more sensitive one of the two ribbon sections a weaker magnetic field than it supplies to the other of the two ribbon sections. By properly selecting the magnetic fields for the respective ribbon sections, the effective sensitivities of the two ribbon sections may be made equal in order to produce the desired cardioid characteristic. This can be accom-- plished in a variety of ways. In a preferred embodiment, I separate the pair of pole pieces associated with the velocity responsive section of the ribbon from the pair of pole pieces associated with the pressure responsive section of the ribbon by suitable non-magnetic spacers, a magnet being connected to the latter pair of pole pieces. The non-magnetic spacers provide a reluctance for the magnetic flux so that the flux in the air gap of the velocity responsive section is less dense than the flux in the air gap of the pressure responsive section. If desired, a portion of the flux for the velocity responsive section may be supplied by a second magnet placed across the pole pieces associated therewith. Also, if desired, the non-magnetic spacers may be made adjustable so that the relative field strengths can be varied at will to provide the desired results. In any case, a high overall sensitivity is obtained by reason of the fact that reducing the flux in the gap of the velocity responsive section produces an increase in the fiux in the gap of the pressure responsive section for a given amount of magnet material.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantagesthereof, will best be understood from the following description of two embodiments thereof, when read in connection with the accompanying drawing wherein Figure 1 is a side elevation of a uni-directional microphone formed according to my present invention,

Figure 2 is a front elevation thereof, and

Figure 3 is a fragmentary front elevation of a somewhat modified form of my present invention.

Referring'more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown a uni-directional microphone of the type disclosed in the above identified reissue patent and comprising a pair of ribbon sections i and 3. The ribbon section I is open both at the front and the back whereby it is responsive to the pressure gradient, or velocity, component of a sound wave. A pipe 5 filled with tufts of felt or the like is placed behind the ribbon section 3 and constitutes an acoustic resistance terminating the back of the ribbon section 3 whereby the latter is responsive to the pressure component of the sound wave. The ribbon section I is disposed in the air gap between a pair of magnetic pole pieces 7, 1, and the ribbon section 3 is disposed in the air gap between a pair of magnetic pole pieces 9, 9.

If the ribbon sections I and 3 are of the same length, there results a mismatch in sensitivity when the pipe 5 is added to the system, the pressure responsive section 3 becoming less sensitive. Because of the requirements of fundamental resonant frequency, the full length of each ribbon section should be retained. To compensate for the resulting unequal sensitivity of the two ribbons, it has been customary to shorten the pole pieces i so that a portion of the velocity responsive ribbon section I extends beyond the pole pieces and is, therefore, outside of the magnetic field. The efiect of this is equivalent to inserting a resistor in series with the ribbon section i, as more fully set forth in my above identified copending application. This is undesirable because it reduces the efiiciency of the microphone. In accordance with my present invention, the two pairs of pole pieces I, 1 and 9, 9 are separated by non-magnetic spacing blocks H, ll of brass or the like, the parts being held together in any suitable manner, as by means of brass aligning pins I2 which serially connect to each other the pole pieces of like polarity of each pair of pole pieces, as clearly shown in Figure 2. One or more magnets I3 are connected to the pole pieces 9, 9 for supplying the necessary flux to the two air gaps. In the absence of the non-magnetic spacing blocks II, and with a field structure of proper design, the flux density in both air gaps would be uniform. Since, as was assumed above, the velocity responsive section I is more sensitive than the pressure responsive section 3, it becomes necessary to make the flux in the air gap where the section I is mounted less dense than in the air gap where the pressure section 3 is mounted in order to reduce the efiective impedance of the ribbon section I and thus substantially the same output from each ribbon section. By connecting the magnet [3 to the pole pieces 9, 9 and separating the latter from the pole pieces "I, I by the non-magnetic spacers H, H, less flux will flow throughout the air gap of the velocity responsive section I than through the air gap of the pressure responsive section 3. The length of the two ribbons or ribbon sections I and 3 may then be kept the same. In this way,

.4 the efiective impedance of each of the ribbon sections or conductive elements may be made substantially equal, the total length of each utilized, and the overall sensitivity of the microphone increased.

In some cases, it may be found desirable to vary the flux density in one air gap relative to that in the other. For this purpose, the pole pieces may all be made somewhat L shaped, as shown in Fig. 3. The lower ends 111 of the pole pieces I are then connected to the adjacent, or upper, ends 9a of the pole pieces 9 by means of screws 15 or the like or brass or other non-magnetic material so that the pair of pole pieces 1, 1 may be made adjustable relative to the pair of pole pieces 9, 9 and the reluctance therebetween varied. In this way, the strength of the field in the two air gaps may be varied relative to each other to provide equal sensitivities for both ribbons, and maximum sensitivity may be obtained consistent with the desired directional characteristic.

From the foregoing description, it will be apparent to those skilled in the art that I have provided an improved uni-directional microphone which has greater sensitivity than a conventional microphone of this type. Although I have shown and described two modifications of my invention, it will be apparent to those skilled in the art that many other modifications, as well as changes in those described, are possible. I, therefore, desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In a uni-directional microphone having a sound wave pressure responsive conductive element movable in a magnetic field and a sound wave pressure gradient responsive conductive element movable in a magnetic field, said pressure gradient responsive element being normally more sensitive than said pressure responsive element when subjected to magnetic fields of the same flux density, the method of compensating for the difference in sensitivity of said elements which comprises reducing the intensity of the magnetic field for said pressure gradient responsive element to a point such that the efiective sensitivities of both said elements are substantially equal.

2. In a uni-directional microphone, the combination of a conductive element responsive to the pressure gradient component of an acoustical wave, a conductive element responsive to the pressure component of said wave, and means providing for eachof said elements a magnetic field, said means being constructed and arranged to provide for said first named element a field of smaller flux density than for said second named element such that the effective sensitivities of both said elements will be substantially equal.

3. In a dynamic microphone, a plurality of pairs of pole pieces of magnetic material, the individual pole pieces of each pair being spaced from each other to provide an air gap therebetween, non-magnetic means spacing each of said pole piece pairs from the other pairs, and vibratile, conductive means movably mounted in said air gaps in association with said pole pieces.

4. In a dynamic microphone, a plurality of pairs of pole pieces of magnetic material, the individual pole pieces of each pair being spaced from each other to provide an air gap therebetween, non-magnetic, variable means spacing 5 each of said pole piece pairs from the other pairs, and vibratile, conductive means movably mounted in said air gaps in association with said pole pieces.

LESLIE J. ANDERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 19,115 Olson et a1. Mar. 13, 1934 474,008 Gibboney May 3, 1892 Number 726,234 1,132,016 1,414,105 1,863,308 1,893,892 1,920,230 2,141,420 2,145,490 2,164,157

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